360

PRECAUTIONS IN CRUSHING

to enter the machine. On the other hand, as the larger machines
require much more power, it is not economical to use a machine
which will deal with pieces far larger than are likely to be supplied
to it. If the pieces too large to be crushed by the machine do not
exceed 3 or 4 per cent of the whole output, it is usually cheaper to
crush them by hand, or even to reject them, than to use a very
large crusher. This is a matter which depends so much on local
conditions that no general rules are of much value.
It is also wasteful to overload a crusher, as this causes clogging
and results in loss of time and a low output. It should therefore
be avoided as much as possible by the use of a sufficiently large
machine. Overloading may often be avoided by the use of a screen
or grid which separates all the material which is of the size of the
crushed product and so does not need to go through the crusher.
A suitable automatic feeding device is also invaluable, in some
cases, for preventing overloading.
In order to avoid an excessive amount of dust from the crushers,
it is advisable to employ chutes to deliver the raw material gradu-
ally into the crusher. These chutes may be perforated so as to
act as screens and separate the small material, which can then by-
pass the crusher.
If the dust from the crusher is very objectionable, it may be
diminished by means of a series of fine water sprays or steam jets,
which will damp the material slightly ; an excess of water should,
however, be avoided.
In the preliminary crushing the rock should be reduced to about
the size of walnuts (i.e. |-1 in. diameter), as pieces about this size
may be dealt with satisfactorily in almost any kind of fine grinding
mill without serious loss of power, whereas the same mill when
supplied with pieces 2 in. or more in diameter may easily require
25-50 per cent more power. It is not, however, economical to
reduce the whole of the rock to powder in the crushers, as the
amount of power required would be excessive. The number of
crushers and their sizes which will give the best results will depend
on the size of the stones and on their nature, as hard stones will
require more crushing and will therefore result in a lower output
per machine than can be obtained with softer stone. In the sand-
working and allied industries it is seldom economical to use a
crusher which will accommodate pieces larger than 14 in. in their
longest dimension, as such pieces usually form only a small propor-
tion of the whole material and are preferably broken by hammers ;
where they are sufficiently numerous, however, larger crushers may
be needed to deal with them. For economical crushing it is usually
advisable to reduce from the largest size which the crusher will
take down to about 4-in. cube in the first, from this size to 2-in.
cube in the second crusher, and from 2 in. to J in. in the third
crusher. Where only two crushers are employed, it is possible to
reduce pieces 4-in. cube to about J-l in. at one operation, though
when the output is sufficiently large to employ two crushers and a